Implanted medical device/external medical instrument communication utilizing surface electrodes

ABSTRACT

A medical device communications system uses subthreshold pulses, modulated to provide relatively high speed electrical communications with inexpensive external devices connectable to a body with the implant by surface leads.

FIELD OF THE INVENTION

This invention is related to inter-device communications between medicaldevices and most particularly to systems that employ sub stimulationthreshold pulses for such communications.

BACKGROUND

The high cost and general level of difficulty in communicating with animplanted medical device using a low cost external instrument hasprevented widespread usage of the data which is currently available frompacemaker and other implantable medical devices to augment traditionaltranstelephonic home follow-up.

Health care systems are increasingly emphasizing and rewarding thoseproducts which reduce the cost of obtaining, communicating, and managingpatient data. Therefore inexpensive devices for remotely monitoring theessential status of pacemaker patients and patients with otherimplantable medical devices is highly desirable. Even small improvementsmay have significant economic and medical benefit.

Difficulties arise in transferring large amounts of data between animplanted medical device and external monitors or other medicalcommunications systems. Telemetry using RF or E fields and H fields iscommonly practiced in, for example, the field of implantable devicessuch as pacemakers and defibrillator/cardioversion devices incommunicating information between the implant and the externaltransceiving device for example, a programmer. This has limitations aswell, primarily on the cost for the external device which goes upconsiderably if it needs to receive telemetry. Also, the energy cost oftransmitting information from the implanted device to outside thepatient's body is higher than using subthreshold electrical pulses andthis therefore depletes the implant's battery, weighing against usingtelemetry too. The overriding consideration for employing externaldevices to receive data through skin contact electrodes is thesimplicity and low cost of the one-way (receiving) device. (Thereceiving device could even be worn like a wrist watch and receivesubthreshold communications for later retransmission).

Therefore to enable better device transmitted communications as the dataamounts and transfer rates are desirably increased, a communicationsprotocol and implementing hardware that facilitates such communicationshas been developed and is the subject of this document.

A list of references where similar or related inventions in the same orother unrelated fields were contemplated follows, and is incorporatedinto this disclosure by this reference thereto.

Davis et al. U.S. Pat. No. 5,544,661, Spinelli et at. U.S. Pat. No.5,413,593, Coppock et al. U.S. Pat. No. 5,503,158, Yomotov, et al. U.S.Pat. No. 5,313,953, Fujii et al. U.S. Pat. No. 5,411,535, Nappholz etal. U.S. Pat. No. 5,113,869, Nolan et al. U.S. Pat. No. 5,404,877,Prutchi et at. U.S. Pat. No. 5,556,421, Funke U.S. Pat. No. 4,987,897,and Strandberg U.S. Pat. No. 4,886,064.

Additionally the Cardiac Telecom HEARTTrac(tm) cardiac monitoring systemmay provide additional information about such communications but at thisdate the inventors have not had an opportunity to review this matter.

There still is a need for a very inexpensive method of getting largeamounts of data from an implanted device to an external device that isas yet unsatisfied by this art. This is especially true in rural areasand in places where sophisticated telemetry systems may be difficult touse or obtain.

SUMMARY OF THE INVENTION

In general this invention provides a way for an implantable medicaldevice to communicate a limited amount of stored data or sensor orstatus data such as battery status and lead condition to an inexpensiveexternal instrument. Additionally it would be an advantage to be able toalso transmit marker data for electrocardiograms. Rather than relying onthe more traditional telemetry communications channel which requires alarge amount of support circuitry and so forth, we are using certainsubthreshold electrical pulsing capability present in some currentimplantable medical devices for this purpose. This subthreshold pulsingmay be delivered along different pathways for minute ventilation, leadimpedance, and capture detection, as well as for this new communicationspurpose. In a preferred embodiment this circuit 10 outputs pulses atrates up to 125 Hz. By modulating a series of such pulses we can easilysend data at 10 to 100 bps or even higher data rates. Preferably,communication occurs on a dedicated set of such pulses.

The pulse train can be by modulated to include data in several ways. Theform (its amplitude or width for example) of the wave of thecommunications pulse may be varied in discrete steps. Including oromitting pulses at a given time in a segment length of time canrepresent various forms of data. Pairing of pulses to send a data bitmay be employed. For example, a zero (0) bit could be represented by apulse followed by a missing pulse, while a one (1) would be representedby a missing pulse followed by a pulse. By limiting ourselves to havingat least one missing pulse every two pulse locations, we eliminate thepossibility of a 00 or 11 configuration and enhance reliability inreading and allows for easier synchronization by this limitation too.Again, since it is so much less costly we make the communication be onlyone way. However, so that the implanted device is not communicatingconstantly to a turned off or disconnected receiver, it is alsopreferable to trigger a communications episode or session from externalto the implanted device. This can be done with a simple “telemetrysystem” or a substitute for one like a magnet and an internal reedswitch that is in the implant device circuitry and which when triggeredby the presence of the magnet, begins a communications episode. (Ofcourse, if a more sophisticated external device is used this subthreshold communication may run simultaneously with or be triggered bythe H or E field telemetry. But the preferred embodiments will usesimple triggers like sounds or magnets or externally applied electricalpulses, or a short burst of H or E field signal produced by aninexpensive external trigger device.) More specifically, each pulse isadapted to avoid pacing, or any tissue stimulation, and to avoid orminimize its effect on the lead to tissue interface. The size of theelectrical pulse energy is therefore below the threshold required forcardiac or skeletal muscle stimulation. These pulses can be safelyapplied by a pacemaker electrode in a pattern which makes them easilyand reliably detectable and interpretable by a simple external device.

A few modifications to currently known devices for deliveringsubthreshold pulses allows for delivery of modulated pulses. A simpledetection algorithm can be implemented in external receivers whichnormally read electrograms of the patient by use of skin electrodes. Thedata read can be translated, error-checked, or otherwise modified totransmit the data to the external device. The external device can storethis or transmit it to other devices or employ it directly to displaydiagnostically useful information or device related information forattending technicians or physicians.

In general then the invention is a communications system forcommunicating between an implanted medical device and a device externalto a living body containing said implanted medical device whereincommunications of data from within said implanted medical device to saidexternal device is accomplished by a communications circuit forproducing modulated biphasic subthreshold pulses in a pattern ofmodulations predetermined to represent data and insufficiently energeticto cause a physiologically significant reaction in living body tissue,and wherein said modulated pulses are transmitted across two electrodeselectrically connected to said implanted device, said electrodeslinkable in an electrical circuit from said communications circuitthrough tissues of said living body, such that said transmission can bereceived by an external device through a plurality of electrodesconnected to said external device when such external device electrodesare in contact with the surface of said body, and the modulations ofsaid subthreshold pulses will be at least one of the set of modulationscomprising (adjustments to timing between delivery of pulses, changingamplitude of pulses, absence of a pulse or pulses in a train of pulses,altered or alternating polarity of pulses, and alterations in pulsewidth).

It has a medical information device for receiving modulated biphasicsubthreshold electrical pulses in a pattern of modulations predeterminedto represent data and insufficiently energetic to cause aphysiologically significant reaction in living body tissue throughelectrodes for affixation to a living body surface having a detectingcircuit for detecting said subthreshold pulses through said electrodes,comprising an amplifier circuit connected to said electrodes andproducing an amplified output signal representing an electrical waveformcomposed substantially of said modulations of said pulses, and having adetecting circuit output for sending said amplified output signal, adecoding circuit comprising a circuit for reading each pulse modulationin said representation of the electrical waveform sent on said detectingcircuit output, and for determining a data bit pattern representing datadecoded from said modulations in said representation of said electricalwaveform, and a conversion circuit for producing a signal representativeof the useful information in said bit pattern.

In one preferred form, the decoding circuit determines one data bitvalue based on based on whether a paired sequence of pulses is in apresent-then-absent order, and an opposite data bit value based on anabsent-then-present order, in another, the decoding circuit determines adata bit value based on the order of the polarity of a biphasic pulse,in yet another, the decoding circuit determines a data bit value basedon whether a biphasic pulse is relatively wide or narrow, and in stillanother form, the decoding circuit determines a data bit value based ona measure of relative amplitude of a biphasic In fact, the decodingcircuit could determine data bit values based on a combination ofmodulations in said subthreshold pulses.

The useful information communicated can represent marker channelinformation, data representing physiologic data about a patient orinformation about a device sending the subthreshold communications fromwithin a body.

The system operates via a method for communicating between animplantable medical device and an external device, starting with somedata within an implantable medical device, sending a triggering signalto an implantable medical device, activating said implantable medicaldevice in response to said triggering signal so as to encode and send amodulated set of subthreshold electrical pulses from said implantabledevice in accord with a protocol having for each data packet a headerfollowed by substantive information, receiving the subthreshold pulsesacross a pair of electrodes on the surface of the body and decodingmodulations of said subthreshold pulses so as to produce a data outputrepresentative of the data transmitted by the implantable medicaldevice.

Preferably, the encoding further adds in error correcting code data tothe modulated subthreshold pulses in each packet.

On the other side of the communications system is the implantablemedical device which has a memory for storing data to be transmitted toan external device and a communication circuit for transmittingsubthreshold signals representing data stored in said memory acrosselectrodes external to but electrically connected to the communicationscircuit, wherein said communications circuit has a generating circuitfor producing a biphasic pulse having a modulatable characteristic, saidproducing circuit adapted to configure each biphasic communicationspulse in a pulse train in accord with a value represented by amodulation information signal, a conversion circuit for providing tosaid generating circuit said modulation information signal to controlthe modulation of said biphasic pulses, and a configuration circuit fortranslating data signal values from said memory into modulation signalvalues for sending said modulation values to said conversion circuit ina stream of values representative of an encoded translation of said datavalues in said memory. It should also have a trigger circuit forreceiving a trigger signal from outside a body and for producing aninternal trigger signal on such an occurrence, and an initiation circuitto receive said internal trigger signal from said trigger circuit and onsuch receipt to initiate program control of functioning of saidgeneration, translation, and configuration circuits so as to send astream of translated, converted and modulated biphasic communicationspulses across said electrodes. In one embodiment, the present inventionis a system that includes an implantable medical device having a canwith surface electrodes positioned for contact with patient tissue. Thesystem also includes a pair of stimulation electrodes for connection topatient tissue and a pulse generation circuit inside the can. The systemfurther includes an electrode switching circuit that is coupled to thepulse generation circuit and delivers electrical stimulation pulsesproduced by the pulse generation circuit that are above a patient tissuestimulation threshold to the pair of stimulation electrodes as therapyto a patient. The electrode switching circuit also delivers subthresholdpulses produced by the pulse generation circuit to the can surfaceelectrodes in a predetermined pattern of modulations constituting anencoded data signal that propagates as a signal transmission through thepatient tissue. The system also includes a control circuit that iscoupled to the pulse generation circuit and the electrode switchingcircuit, causes the pulse generation circuit to selectively generate thestimulation pulses and the subthreshold pulses, and causes the electrodeswitching circuit to selectively apply the selectively generated pulsesto the pair of stimulation electrodes and the can surface electrodes.The system also includes a plurality of electrodes adapted to beelectrically connected to a patient's skin to receive the subthresholdpulses transmitted through the patient tissue. In addition, the systemincludes an external device coupled to the skin electrodes to detect theencoded data signal.

Of course, any communications to the external device could be done so asto later be sent by the external device across a telephone or othercommunications network to a medical information group located at adistant receiver.

Numerous other features and advantages are described with reference tothe following drawings.

FIG. 1 is a graph of a generalized biphasic pulse for use with thisinvention.

FIG. 2 is a graph of four broken segments showing pulse modulationfeatures in accordance with preferred forms of this invention.

FIG. 3 is a heuristic block diagram.

FIG. 4 is a graph of a preferred biphasic pulse for use with thisinvention.

FIG. 5 is a graph of a pair of preferred biphasic pulses for use withthis invention

FIG. 6 is a graphic drawing of a pulse stream in accord with a preferredform of this invention.

FIG. 7 is a graphic representation of a timing diagram with cardiacevent features and time periods highlighted as features thereon.

FIG. 8 is a graphic representation of a timing diagram like FIG. 7.

FIG. 9a is a simplified block diagram representing the features animplanted device may have in a preferred form of the invention.

FIG. 9b is a simplified block diagram representing the exterior (outsideof the patient) device as would be used with a preferred form of thisinvention.

FIG. 10 is a block diagram representation of a protocol.

FIG. 11 is a block circuit diagram.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The basic pulse waveform is shown in FIG. 1 by line 10. In general itcan be described by an amplitude expressed in voltage (V_(pw)) andeither side of the biphasic pulse can define a specific time period(T_(pw)). (For physiologic reasons, the net energy delivered to themuscle must be zero.) In order to code data using these pulses,characteristics of each individual pulse may be modulated, therelationship between pulses may be modulated, or some combination oftechniques used. Each modulation technique can be used to includemultiple bits per pulse to raise the transmission rate. Combiningmultiple techniques can additionally raise the information transferrate.

FIG. 4 describes a single subthreshold waveform 40 having been positivepeak at 41 and a negative peak at 42, and timing measurements describedby arrows 43-48.

FIG. 5 is being graph 50, of two adjacent pulses 51 and 52, having thesame time measurement values on pulse 51 and additionally describes anew time parameter illustrated by arrow 53.

Individual pulses can vary by width (T_(pw)) or amplitude (V_(pw)).Additionally by choosing complementary pair electrode hardware (e.g.delivering the pulse, tip to can first and then can to tip in apacemaker configuration), the polarity relationship of the pulse phasescan be changed from positive/negative to negative/positive. Suchvariations as just described are illustrated in FIG. 2. Note on thefigure the changes in height of the pulses on line 3, the width of thepulses on line 4, the change in the order or phase polarity on line 5 ofthe pulses, and the one combined form of amplitude, width, and polaritymodulation in line 6. The interval between pulses can also be used toinclude data bits of information. The repetition rate (pulse frequencymodulation) or missing pulse configuration in a constant rate pulsetrain (another form of pulse frequency modulation) may be used to encodeinformation into the pulse stream of subthreshold pulses.

Also, missing pulse modulation is difficult to combine with themodulations illustrated in FIG. 2 since pulses would frequently beomitted in the pulse stream. This would make interpretation difficult.In general the specific technique or techniques must be decided by theuser of this invention as a result of considering trade-offs betweenincreasing data rate with more complex demodulation, lower cost externalinstruments, and achieving a specific level of reliability.

For the purpose of explanation, we describe a simple frequency pulsemodulation scheme that is easily decoded and produced, employing aconstant space between times for pulses to possibly occur and theabsence or occurrence of a pulse during such times indicating a data “1”or “0”. However we also describe how to enable numerous other modulationschemes which employ the available features of the subthreshold pulse wecan deliver. One of ordinary skill in this art can employ the heuristicprinciples described with reference to the simple frequency modulationscheme we describe to the other forms of pulse modulation availablewithout difficulty. The designer of a device in accord with thisdisclosure will have to consider that the more complex the modulationscheme employed, the more complex and expensive the receiver willprobably have to be. Accordingly it is expected that the person ofordinary skill have some knowledge of the use of biopotentialamplifiers.

FIG. 6 illustrates an 8 millisecond segment 20 of information under thissimple modulation scheme. In this space of 8 mS, 5 pulses can be sent,but only 4 are, pulses 10 a-d. (The segment may also be considered 10 mSlong if you include the full time for the fifth pulse to end before the6 the pulse may be allowed to occur). The time between each same sizedand polarity simple biphasic subthreshold pulse is expected to be (forthis modulation scheme) 2 mS. There is one space for a pulse missingbetween pulse lOc and lOd. Thus, the segment would be read, in itssimplest form as a digital data stream of 11101. As is well known tothose of ordinary skill in the communications art, this could be a partof a series of allowable pulse configurations, for example where onlyone “zero” is allowed and must be positioned in either the second orfourth position, thus yielding only three bits of information from afive pulse long code. Such pulse code schemes are used to enhancethrough redundancy the ability of a receiver to guess at the correctdata where, for example one or another pulse might be lost in a noisyenvironment. We use checksum data for example, but other redundanciesand well know schemes used for the same purposes should be considered tobe within the ambit of this invention.

Another preferentially designed feature is the limitation oftransmission times to segments of time related to a sensed cardiac eventor a pacing pulse. (Use of this feature is preferred especially wherethe receiver cannot distinguish communications pulses from physiologicsignals very well, or more importantly, where there may be doubt aboutthe inability of the communications pulses to trigger physiologicreaction in the patient's cells). FIG. 7 illustrates such a preferredembodiment. The segments such as segment 20 of FIG. 6 are limited intime to a period wherein the tissue is refractory to responding tostimulation after the delivery of a pulse (A or VP point on the line 25)or after an equal amount of time following a cardiac natural event (A orVS). These preferred transmission time periods are referenced withnumerals 21 and 22.

Referring to FIG. 8, there is a single channel transmission in theatrial channel having marker channel information in each transmission.The atrial channel is on line A and the ventricular channel is line V.In this illustrated scheme, a redundant transmission occurs after anevent, here VP2, was triggered by the occurrence of an Atrial Senseevent AS. Since it occurred in the transmission frame of the Amarker D3,the same Amarker data will be retransmitted.

Such periods are chosen to be set for the period of time the cardiactissue is refractory to stimulation, thus even communication ofsubthreshold pulses near the stimulation threshold for the tissue willnot cause a depolarization. These times of absolute tissuerefractoriness are well known in the pacemaker art.

When used in this manner, simple but powerfully descriptive makerchannel information can be transmitted. (The seminal disclosureregarding marker channel information generally is U.S. Pat. No.4,374,382 issued to Markowitz and incorporated herein by thisreference). Thus, for a 24 bit data stream in each space following apacing pulse the available amount of information for transmission usingour simple preferred scheme for modulation is 2₂₄ messages. Thus, thereceiving device could have a lookup table with 2₂₄ entries, which couldbe used for transmitting that much information regarding the presentstate of the implanted device, it's history, the patient's physiologicalevent history and in fact, any data usefully used outside the body wherethe implant resides. It is of course, important to recognize that withthe inclusion of framing and error checking information as integralparts of the bit stream, substantially less than this amount of datawill be available. Thus, the size of the table could be reduced toinclude spacer information, headers, or other redundancies to ensurecorrect receipt of the intended transmitted information, as might bedesigned into the table by one of ordinary skill. Or, the protocolinformation can be used by a preprocessing circuit or program to sendthe remaining substantive data to the table look-up circuit or program.The receiving device could use this information to print maker channelinformation on the moving electrocardiograph it is making, and/or storethe information for later retrieval or transmission to a more empowereddevice where the information can be interpreted for diagnostic orresearch purposes.

In our preferred embodiment, we developed a specific integrated circuitfor varying the parameters described across a range of values in aseries of discrete steps. See Table 1 below for these values. A designerof systems employing this invention can make changes in these selectionsand ranges within the ambit of this invention so long as the changescontinue to provide distinguishable features for the receiver and solong as the pulses are modulated to remain below the threshold whichwould adversely affect body tissue through electrical stimulation.

Just to detail the clear implications for data transmission again; witha simple modulation scheme as we are detailing here for a preferredform, for example, using a single binary modulation at 2 mS/pulse area,the data rate is about 50 bits per second; or using a similar singlepulse modulation scheme such as phase polarity at 125 bits per second,thus the raw data or bit rate is limited to 125 bps. By using some ofthe independent modulation schemes described above, nine bits per pulsecan easily be achieved with a resulting raw data rate of 1125 bps.However, using such high data rates requires a more sophisticatedreading device to parse the information from the analog encoding ofsmall power signals, and since for the present moment, price is the mainconsideration, the simpler modulation schemes are preferred.

Since this data transmission scheme is for transmitting data frombetween implanted device within a patient's body and an inexpensiveexternal device similar to an electrocardiogram receiver/recordingdevice, some type of redundant transmission information is useful toensure good transmission of data through noisy environments and lessthan ideal conditions. Redundancy is also important because there islittle or no opportunity to inform the implant that its data is notunderstood, even if the inexpensive receiver could determine that thedata is not good by itself. Multiple transmissions of the same data,and/or various forms of error correction are both classes of useableredundancy that may be employed for this. In one preferred embodiment wesend a message having an error correcting code incorporated into themessage and use a decoding circuit to correct any errors located in themessage. Depending on the complexity of this added redundancy, whichwill need to be included, the amount of data that can be sent in a giventime period will be reduced by from about 5 to 70%.

In another preferred embodiment, we transmit data continuously once thetransmission is activated without regard to refractory periods since thesize of the pulses is too small to stimulate the tissue response. Inthis preferred embodiment, much more data can be transmitted in the sameperiod of time since we don't have to wait for refractory periods.

Specifically, in our preferred example embodiment, information istransmitted as words that are 24 bits in length. We could design this innumerous ways, but for marker channel information a word ofapproximately this length or shorter should be used if transmission timeis limited to refractory cardiac times, and is using something close indate rate to the example modulation scheme. Our preferred marker channelwords are 21 bits in length. A word can represent data file header, datafile segments, or marker channel information. For unipolar leadconfiguration one word is transmitted per pacing cycle. For bipolar leadconfiguration up to four words are transmitted per pacing cycle. Markerchannel information is transmitted with one word per pace or senseevent. All bits within a word need to be transmitted withoutinterruption. If the transmission of a word is interrupted the entireword must be retransmitted at the next available opportunity. Apreferred data structure for the transmitted word is as shown.

It should be noted that where the implanted device has no concern aboutthe potential to stimulate tissue, say for example, because it is merelya subcutaneous implant monitoring a local physiologic conditionincapable of sending large stimulation pulses, than much longer/shorteror just different data structures could be used, as will by now beapparent to the reader. Additionally, the localization of the externalelectrodes near the subcutaneous device would obviate any concern aboutisolating the communication pulses from physiologically producedelectric signals.

Data Bit Data File Header Notes D0, D1 11₂ Used for clocksynchronization. D0 is first bit of word D2-D6 00001₂ Indicates thatthis is a data file header D7-D8 00₂ to 11₂ Used to identify up to 30data file segments as a group. D9-D13 00000₂ to Indicates the number ofdata file segments 11111₂ to be transmitted in a group. D14 CompleteTransmission complete indicator, 1 indicates that this is the last groupto be transmitted. D15-D18 00000₂ to Unassigned data file header bits.11111₂ D19-D23 ECC Error detection and correction information. Data FileData Bit Segment Notes D0, D1 11₂ Used for clock synchronization. D2-D600010₂ to Used to order data file segments for 11111₂ reconstruction ofdata file. 30 segments can be ordered. D7-D18 Data (12 bits) Data fieldfor a data file segment D19-D23 ECC Error detection and correctioninformation. Data Bit Marker Channel Notes D0, D1 11₂ Used for clocksynchronization. D2-D6 00000₂ Indicates that this is marker channelinformation D7 Lead 0=Atrial lead, 1=Ventricular lead D8 Event 0=Senseevent, 1=Pace event D9 Sense type 0=Non refractory sense, 1=Refractorysense D10-D15 Correction The number of 10 ms10mS periods prior to thefirst bit of this frame that the marker event occurred. Result roundedto the nearest 10 msmS. D16-D20 ECC Error detection and correctioninformation.

In one example embodiment that limits transmission to refractory periodsbut includes marker channel information, data files consist of a dataheader file and up to 30 data file segments. Such segments can be brokenacross the refractory periods used in the marker channel transmissiontimes if desired, but this may result in a slower transmission of largeamounts of data. On the other hand, by only transmitting in therefractory period, the implanted device is assured of not capturing thecardiac tissue. All information is transmitted twice to allow for therecovery of missed information. If the reading device is expecting theinformation after the pace (or sensed event) pulse, there is no need fora header. Similarly, marker channel information does not require aheader. Marker channel transmission occurs once per event. Incorrectinformation that can not be corrected with the checksum information willbe discarded by the receiver. The data file is constructed as shown:

Cumulative Transmission File Transmitted Pace Event Time at 85 BPMUnipolar leads Data header file (group 0) 1 Data header file (group 0) 2Data file segment (0) 3 Data file segment(1) 4 Data file segment(n), n ≦29 32 for n=29   23 seconds Data file segment (0) 33 Data filesegment(n), n ≦ 29 64 for n=29   45 Seconds Bipolar leads (Example showstwo groups transmitted) Data header file (group 0) 1 (Transmissioncomplete bit = 0) Data header file (group 0) 1 Data file segment (0) 1Data file segment(1) 1 Data file segment(n), n ≦ 29  8 for n=29  5.6seconds Data file segment (0) Data file segment(n), n ≦ 29 16 for n=2911.2 Seconds Data header file (group 1) (Transmission complete bit = 1)Data header file (group 1) Data file segment (0) Data file segment(n), n≦ 29 Data file segment (0) Data file segment(n), n ≦ 29 32 for n=29 22.4Seconds

Additionally one may wish to employ a more detailed protocol. An exampleprotocol for data communications is described with respect to FIG. 10wherein a Marker Frame 101 and a Data Frame 102 structure can coexist ina single transmission. Here the data file bit stream 103 is brokenacross the two frames 101 and 102 and it resides in chunks of thesegment data within the protocol marked Segment #1-N. A synchronizationportion 105, a marker space which is zero in one frame and one in thenext to distinguish one frame form another 106, segment number or markertype 107 marker time correction data 108 error correcting code 109, andfinal synchronization space 110, transmitted in the order shown, make upthe overall protocol, allowing for easy decoding by a compatible readingdevice. In a segment having other data than marker data such as frame102, the segment 108 contains the data. One of ordinary skill in thedata communications art will be able to produce innumerable protocolarrangements and the specifics are best left to the designer of thespecific devices. Error correcting codes are well known in that field aswell. See for example, Error Control Coding: Fundamentals andApplications by Lin and Costello, Prentice Hall, Inc., Englewood Cliffs,N.J., Copr. 1983, and Error Correcting Codes by Peterson and Weldon, 2ndEdition, MIT Press, Boston, Copr. 1972.

The preferred circuitry is described in overview with reference to FIG.3. A bus 31 connects a microprocessor 32 with the memory 33 and thepulse generator and measurement circuit 34 which develops thesubthreshold communications pulses (as it also can develop othersubthreshold pulses for purposes such as determining minute ventilationthrough impedance measurements as was described in U.S. Pat. No.4,702,253 issued to Napholtz, among others. Such pulses can have otheralternative uses as well which may be employed by the same circuitry forgenerating these pulses any time they are not being used forcommunications as they are for this invention). A microprocessor orother control circuitry 32 formats a set of register values to be sentto the excitation control register. These register values set theparameters of each individual pulse and its timing to include thedesired data values and redundancy. To start communication, themicroprocessor writes the first value to the control register underfirmware control. Subsequent values are automatically transferred frommemory to the control register by either the microprocessor or a thedirect memory access (DMA) controller circuit in the microprocessor. Aprogram in memory may control the processor circuit 32 to encode thedata sent with the appropriate conversions to the transmission code andinclude any protocol features that may be required. Microprocessor andprogram control are the most flexible way to set this operation up,however one could use fixed analog circuitry to avoid use of registersand other memory devices if desired, but that would not be preferred.

An example preferred excitation control register 125 is shown in FIG.11. As it is well known how to convert values in a register to signalvalues to modulate a waveform no detailed description is provided here.It is sufficient to say that a larger number of elements(125_(1 . . . n)) provides more flexibility in range between the twopolar values of a given pulse modulation characteristic (such asamplitude or pulse width). But since in our preferred embodiment we onlydetermine whether a pulse or non pulse condition will occur at the timefor a next pulse during a communication, the flexibility provided bysuch a register is surplussage for this simple embodiment. If howeverone prefers to enable more forms of modulation, the diagram of FIG. 11should be referenced. There, the value in register 125 would program anoutput circuit 126 to produce the pulse modulated for thecharacteristics defined by the data in the register 125.

FIG. 9a represents the shell of the implanted device in dotted line 40,here having two surface electrodes 47 and 48, electrically isolated fromeach other. There may also be electrodes such as an indifferentelectrode employing the exterior metal can or housing 14, and electrodes16 a 16 b 17 a and 17 b on leads located so as to provide stimulationwithin specific tissues, as illustrated here, in a heart right atrium RAand right ventricle RV. These devices could be pacemakers,cardioverter/defibrillators, drug pumps, or any implanted device whichcan generate subthreshold pulses for communication in accord with thisdescription. The form of the implanted device is relevant to the choiceof modulation and data transmission schemes as has been explainedthroughout this document. For example, a simple two to four electrodesubcutaneous electrocardiogram recording device has no chance ofaccidentally causing physiologic changes in tissue during use of thecommunications pulses, so continuous rather than only refractory timecommunication would be preferred. The systems with more electrodechoices may be used to enhance the signal received by the reading devicethrough experiment and the preferred transmission set of electrodes maybe fixed at the time of implant.

In FIG. 9a, only the relevant features of a typical implanted devicewhich could be used with this invention are shown. The pulse generatorcircuit 74 creates the waveform pulse and sequence of pulses in accordwith parameters written by the microprocessor 75 under program controlto the control register CR of circuit 74. The Microprocessor 74 maytransfer these parameters through a DMA circuit or across bus 18. Theoutput of circuit 74 is applied to the electrode switching circuit 71 inaccord with the preferred sending path to the selected electrode pair.The configuration of the switches in circuit 71 is determined by valuesin its control register (not shown) which are in turn selected by themicroprocessor under program control. The data communicated willgenerally reside in a specific area of the memory circuit 10, havingbeen stored there by the implanted device during its normal operationfor this purpose. The application of the waveform pulse across a pair ofelectrodes causes a current to flow and be detectable by an externalreading device via electrodes affixed to the skin of a patient. Theinitiation of a communications session as just described is preferablyperformed by the activation of some internal switch such as a reedswitch or Hall-effect sensor by a magnet placed near the implanteddevice, or by some kind of telemetered wakeup signal generated by aprogrammer or a simple activator device capable of transmitting a simpleactivation sequence. this function is illustrated here by the use of a“telemetry” block in dotted line within the shell 40 of device 41.

FIG. 9b. illustrates an external reading device 60 connectedelectrically to a patient's skin SK by electrodes PE1 and PE2. Thesignals received by these electrodes (which could be any combination ofknown electrocardiogram type electrodes) is fed into a receiving senseamplifier circuit 62, and commonly will produce an analog display of anelectrocardiogram 64 which represents the varying signal value foundbetween any two of the leads on the patient's body. Additionally, theinput signal is sent to a decoding circuit 63 that detects the bitstream in any of the manners described above, depending on the design ofthe reading device 60. The data from that stream is fed to a memory andoutput management circuit 65 for storage and use through communicationscircuits 66 or by adding to the display or printing an additionaldisplay via circuits 67, if desired. Additionally the data may bereceived in a coded format that requires a decoder circuit to do errorcorrecting and accommodation to redundancies or intradata modulationtechniques. Further a microprocessor circuit 68 may have a program thatoperates on the received data to perform diagnostic or other reportingfunctions, and a telephonic transmission or other transmission circuitmay send the relevant data received and/or digested by the programs tosome other devices for further use. Commonly a programmer device 61 willbe a receiving device for such information and may perform additionaloperations on the data. The trigger for the transmission by the device40 may be from an attached or separate trigger device 86, here a simplemagnet, which acts upon the circuit 77 in an appropriate manner to thecircuit 77 design. A separate programmer device 61 could also providethe trigger to start the transmission by the implant 40.

What is claimed is:
 1. A system, comprising: an implantable medicaldevice having a can with surface electrodes positioned for contact withpatient tissue; a pair of stimulation electrodes for connection topatient tissue; a pulse generation circuit inside the can; an electrodeswitching circuit coupled to the pulse generation circuit and deliveringelectrical stimulation pulses produced by the pulse generation circuitthat are above a patient tissue stimulation threshold to the pair ofstimulation electrodes as therapy to a patient and deliveringsubthreshold pulses produced by the pulse generation circuit to the cansurface electrodes in a predetermined pattern of modulationsconstituting an encoded data signal that propagates as a signaltransmission through the patient tissue; a control circuit coupled tothe pulse generation circuit and the electrode switching circuit tocause the pulse generation circuit to selectively generate thestimulation pulses and the subthreshold pulses and to cause theelectrode switching circuit to selectively apply the selectivelygenerated pulses to the pair of stimulation electrodes and the cansurface electrodes; a plurality of electrodes adapted to be electricallyconnected to a patient's skin to receive the subthreshold pulsestransmitted through the patient tissue; and an external device coupledto the skin electrodes to detect the encoded data signal.
 2. Thecommunications system of claim 1, wherein the control circuit includes amicroprocessor.
 3. The communications system of claim 1, wherein thepulse generation circuit modulates the second electrical pulses using amechanism selected from the group consisting of frequency modulation,amplitude modulation, pulse train modulation, pulse width modulation,and pulse polarity modulation.